A variety of systems using data carrier of the electromagnetically coupled type have heretofore been proposed depending upon the communication systems and the power source systems. First, the power source systems can be divided into those of the cell-containing type in which the data carrier contains a cell and the power sourceless type which does not contain the cell but which receives an electromagnetic signal transmitted from the fixed facility and rectifies the signal to obtain a voltage that serves as a power source. The communication systems can be divided into those of the one-way communication type in which the data stored in the data carrier are transmitted to the fixed facility and those of the two-way communication type in which the data stored in the data carrier can be rewritten depending upon the data transmitted from the fixed facility; and the data can also be transmitted from the data carrier to the fixed facility. Moreover, the two-way communication systems can be classified into those of the two-frequency type in which a frequency of an AC magnetic field generated by the fixed facility is different from a frequency of an AC magnetic field generated by the data carrier and those of the one-frequency type which use the same frequency. The present invention is chiefly concerned with a power sourceless two-way communication system which uses only one frequency.
Described below are conventional examples of each of the above-mentioned systems. First, the two-way communication systems to which the present invention is concerned have been disclosed in Japanese Unexamined Patent Publications (Kokai) Nos. 2-291091, 3-273465 and 4-692. There have further been proposed power sourceless two-way communication systems that resemble the present invention, and Japanese Examined Patent Publication (Kokoku) No. 3-12353 discloses the one which is of the two-frequency type. Moreover, those of the power sourceless one-way communication type have been disclosed in Japanese Examined Patent Publications (Kokoku) Nos. 3-19591, 3-12352, U.S. Pat. Nos. 3,964,024 and 4,129,855. In particular, Japanese Examined Patent Publication (Kokoku) No. 3-25832 discloses a communication system of the power sourceless one-frequency one-way type to which the present invention is related. However, all of the above-mentioned prior art documents employ only one fixed facility and one data carrier, and do not give any attention concerning the problem of interference among fixed facilities that develops when the individual fixed facilities communicate simultaneously with the data carriers.
The problem of interference among the fixed facilities which will be considered below is the principal object of the present invention over the aforementioned prior art. First, Japanese Unexamined Patent Publications (Kokai) Nos. 2-291091, 3-273465 and 4-692 simply disclose the idea of two-way communication systems but do not at all concretely disclose the communication systems. On account of their cell-containing system, however, it is considered that these prior art disclosures must employ the two-frequency system. Japanese Examined Patent Publication (Kokoku) No. 3-12353 employs the two-frequency system as described above. In the case of the two-frequency system, the problem of interference among the fixed facilities can be avoided by separating the two kinds of frequencies using different filters, as is well known.
In the case of the one-way communication system, the problem of interference among the fixed facilities does not develop even when it is of the power sourceless one-frequency type (Japanese Examined Patent Publication (Kokoku) No. 3-25832) which resembles the present invention as will be described with reference to FIG. 28.
FIG. 28(B) shows transmitted and received waveforms in a power sourceless one-way communication system disclosed in Japanese Examined Patent Publication (Kokoku) No. 3-25832 of the case where a fixed facility G2 is disposed close to a set of a fixed facility G1 and a data carrier C1 that is shown in FIG. 28(A) in the one-frequency type data carrier system.
In FIG. 28, GTS1 represents an AC magnetic field that is transmitted to the data carrier C1 from the fixed facility G1, i.e., represents unmodulated AC signals for feeding electric power without transmitting data. CDS1 represents data signals of the data carrier C1, and CTS1 represents an AC magnetic field transmitted to the fixed facility G1 from the data carrier C1 and which is obtained by modulating the AC signals of the same frequency as the GTS1 with the data signals CDS1. GTS1-1 represents data signals of the fixed facility G1 which are detected upon receiving the AC magnetic field CTS1. GTS2 represents an AC magnetic field transmitted from the fixed facility G2, i.e., represents unmodulated AC signals for feeding electric power without transmitting data like the above GTS1. GKS1 represents an interference signal of the fixed facility G1 which is obtained by receiving the AC magnetic field GTS2 transmitted from the fixed facility G2 and is detected by the fixed facility G1. Since GTS2 is an unmodulated AC signal, the interference signal GKS1-2 which is detected therefrom is under the condition of no signal. GKS-1 represents total data signals of the fixed facility G1 obtained by adding the detected signals GKS1-1 of AC magnetic field CTS1 transmitted from the data carrier C1 and the interference signal GKS1-2 from the fixed facility G2 together. In the case of the one-way communication system, the AC magnetic field GTS2 of the fixed facility G2 forms unmodulated AC signals, and there exists no interference signal GKS1-2 in the fixed facility G1. Therefore, the total data signals GKS1 have the same waveform as the data signals GKS1-1, and there develops no problem of interference from the other fixed facility G2.
In the case of the one-frequency two-way communication system in which a plurality of fixed facilities G1, G2, . . . , Gv are arranged close to one another, the AC magnetic fields GTS1 and GTS2 from the fixed facilities in FIG. 28 are modulated with data signals. Therefore, the modulated AC magnetic fields from the fixed facilities are received as interference signals by the antennas of other fixed facilities that have a resonance characteristic at the same frequency. As a result, the total data signals of the fixed facility that has received the interference signal become different from the data signals which are obtained by detecting the signals transmitted from the data carrier, and it becomes difficult to read out correct data.
That is, concretely speaking, the system which uses the power sourceless electromagnetically coupled data carrier has been put into practical use only in a manner in which the communication is made one way from the data carrier to the fixed facility. In many of these systems, the AC magnetic field generated by the fixed facility is not modulated and the field of synthesizing the AC magnetic fields established by a plurality of fixed facilities is not modulated, either. Therefore, the fixed facilities do not interfere with each other, and the data communication is not disturbed. In a system of the type in which the AC magnetic field generated by the fixed facility has a frequency different from that of the AC magnetic field generated by the data carrier, the data signals are separated, relying upon the filter technology, since the AC magnetic field generated by the data carrier has a frequency different from the frequency of the AC magnetic field generated by the fixed facilities. This makes it possible to preclude the effect of AC magnetic fields generated by the fixed facilities which are close to each other.
When the data are transmitted to the data carrier by modulating an AC magnetic field generated from the fixed facility, a very great change takes place in the AC magnetic field, causing a change in the electromotive force induced in the antenna coils of neighboring fixed facilities, though they are located considerably far away. Since the frequency of the AC magnetic field generated by the data carrier is the same as the frequency of the AC magnetic field generated from the fixed facility, it is very difficult to distinguish a change in the induced electromotive force from a change in the electromotive force induced by the AC magnetic field generated by the data carrier. It has therefore been considered difficult to read correct data from the data carrier. On the other hand, when the AC magnetic fields generated by the data carrier and the fixed facility have different frequencies, the electromotive forces induced by the two AC magnetic fields can be separated relying upon the filter technology. With the data carrier of this system, however, the AC magnetic field that is output is so weak that it is subject to be affected by external noise and the communication is limited to short distances.
The data carrier system which uses the power sourceless electromagnetically coupled data carrier has not been realized in a form which permits two-way data communication between the data carrier and the fixed facility. The present inventors have developed an electromagnetically coupled data carrier which mounts a nonvolatile memory of high performance which is called C-MOS-IC that consumes very small power and which is devised in a sophisticated manner to rewrite the memory, and have developed a fixed facility for the electromagnetically coupled data carrier as a result of contrivance. The object of the present invention therefore is to solve the above-mentioned problem and to provide a data carrier system which uses an electromagnetically coupled data carrier primarily of chiefly the power sourceless two-way communication type, and in which the AC magnetic field of one frequency is employed for communication between the fixed facility and the data carrier to increase the distance of communication and to read out correct data from the data carrier even when interfered with by modulated AC magnetic fields from other neighboring fixed facilities.